Symptoms
Consider the following scenario:
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You create a database availability group (DAG) that contains three or more member servers in a Microsoft Exchange Server 2010 environment.
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A router or switch issue occurs, and the issue causes intermittent connectivity between the member mailbox servers in the DAG.
In this scenario, the failover cluster for the DAG loses quorum. Therefore, the DAG stops running, and all mounted mailbox databases that are hosted in the DAG are dismounted. When the DAG is unavailable, users cannot access the mailboxes that are hosted in it.Note The expected behavior is for as few mailbox servers as possible to be removed from the failover cluster for the DAG. This is so that the remaining mailbox servers can still communicate with one another. Therefore, the DAG can continue to run if the cluster still has quorum.
Resolution
To resolve this issue, install the following update rollup:
2785908 Description of Update Rollup 5 version 2 for Exchange Server 2010 Service Pack 2
Status
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
More Information
For more information about DAGs, go to the following Microsoft website:
Understanding database availability groups For more information about a similar issue, click the following article number to view the article in the Microsoft Knowledge Base:
2552040 A Windows Server 2008 R2 failover cluster loses quorum when an asymmetric communication failure occurs
